Primary Driven Gear 25 (25/69

The replacement of the primary drive cog on the crankshaft of the small frame motor is one of the most simple and effective methods to alter the overall gear ratio of your motor. The key to precise gear ratio tuning are cogs with straight cut teeth. Due to the extensive range of cog sizes (or number of teeth) you can have the exact overall gear ratio required for your motors particular state of tune.

The DRT gear cog teeth are precisely manufactured from an especially tough grade of steel and then hardened. They are CNC machined from solid steel and are then put through an extra milling process to remove any unnecessary material and so save as much weight as possible.

The 24/72 - 3.00 sized primary drive/clutch basket has proved suitable for tuning cylinder set ups between 75 - 102cc. The gear cogs produced by DRT in the cog teeth sizes 21,22 and 23 fill the gaps in the overall gear ratios for smaller 75 - 85cc cylindered machines. These items are CNC machined from solid steel and then remilled to remove any excess material, to save weight. The gear cogs with 24, 25, 26 and 28 cog teeth are usually the preferred sizes in both circuit and sprint racing in combination with the input shafts produced by DRT. Their MATRIX or SINCRO versions also have their own particular recommended gear cog sizes. The cog with 26 teeth has proven its worth when used with the more moderately tuned, road-going VESPA motor projects with a cylinder capacity of between 125 and 130cc. The 27/69 primary drive gear cog ratio, in all its variations, is the most commonly used when tuning motors with a cylinder capacity of between 130 and 155cc.

The 29/68 primary drive gear cog ratio is usually the best option for high-end performance orientated projects.

It can also be used in the more mildly tuned engine set ups in combination with 26 or 27 teeth gear cogs, making for higher speeds at lower engine rpm. For all you ultimate top-end speed freaks we have a gear cog available with 30 teeth which raises the overall gear ratio of your motor to 2.27!

We have the full details of which overall gear ratios work well with which cylinder classes available for download at our website.

SIP-TIP: These primary drive cogs are best combined with the primary drive transmission units that we also have available produced by RMS, OLYMPIA, SURFLEX or POLINI. Do not forget to also order new shaft-end fixing nuts and safety washers, the relevant woodruff keys, clutch friction plates and fresh gearbox oil for safe and enduring use of your motor.